This invention relates generally to water-soluble cationic copolymers dispersed in water-in-oil emulsions and an improved process for preparing such copolymers. More particularly, the present invention relates to cationic water-soluble polymers which are prepared by a delayed addition process as specified herein and which, due to the method of preparation, exhibit improved performance characteristics.
Water-in-oil emulsions containing cationic water-soluble copolymers dispersed therein are well-known in the art. Such emulsions have found a wide variety of uses, for example, as flocculants in the mining and paper industries and in sewage treatment, and as mobility control agents in enhanced oil recovery. These emulsions have generally been made by dissolving the entire monomer charge in the water and then preparing the water-in-oil emulsion which is the medium in which the polymerization is conducted. This conventional preparation process has been found to generally limit the performance characteristics of the resulting copolymers.
One attempt to overcome this problem with cationic copolymers is disclosed in U.S. Pat. No. 4,152,200 of A. T. Coscia et al. wherein a portion of the cationic comonomer is withheld from the initial emulsion and then, after polymerization has been initiated, is added incrementally throughout a major portion of the polymerization reaction. The intent of Coscia et al. appears to be to balance the reactivity ratios of the monomer mixture to take into account the differences in reactivity of the cationic monomer versus the non-ionic comonomer. However, the cationic copolymers produced by Coscia et al., although usually superior in performance to conventionally prepared polymers, still have insufficient performance characteristics for many uses of cationic copolymers, particularly as retention aids for paper or as flocculant for municipal sewage sludge.
It is accordingly an object of the present invention to produce water-in-oil emulsions of finely dispersed water-soluble copolymer particles in which the emulsions have improved performance characteristics. It is a further object to develop an improved process for preparing cationic copolymers whereby the surfactant and/or oil contents may be reduced below those amounts conventionally used. It is a further object to develop an improved paper retention aid. It is a further object to develop an improved flocculants for sludge dewatering. These and other objects will become apparent from the ensuing detailed description.